well, I wouldn't call a central line life-saving usually lol. The short of it: Internal medicine deals with patients in the hospitals as a hospitalist, or can deal with patients in clinic on an outpatient basis like family medicine doctors. They don't specialize and no longer commonly work in emergency medicine, nor do they work in intensive care units, generally. In the hospital, their job is to refine or correct the admitting diagnosis, (if anything, they're pathophysiology specialists), adjust therapy to meet the patient's daily needs, establish an appropriate discharge plan including appropriate follow-up and appropriate outpatient therapy.
In terms of "shocking" someone or procedures, they may need to put in central lines, but they may not always be the ones to do so. When there is a code on the floor of the hospital, they may be part of the code team and run/direct the resuscitation, but they won't be as skilled at this as an emergency doctor. Mostly because they don't do it as often, and they may ntot even be part of a code team in the hospital. In teaching hospitals, it is the residents who do this with an attending overseeing. I dont' know what it's like in the community setting, but I have a feeling nurses are more the ones who do this.
Emergency Physicians deal with anything and everything that comes into the emergency room, specialize in acute and hyperacute medical and surgical issues, and are well-trained in the art of resuscitation. We do initial diagnosis and management, but in many cases, we are not the ultimate provider. Our job is to figure out what's going on and get them where they need to be in one piece, whether that is home with stitches, referal to an outpatient specialist, transfer to other hospitals, or admission to any of the hospital's inpatient services. We do life-saving procedures, but these are often a bridge to definitive care. If a guy comes in with major traumas, we're going to find out if there's internal bleeding, we'll insert breathing tubes, we'll save someone's life temporarily by shoving in a chest tube or draining fluid from around the heart if we need to. And then we'll get them to a trauma surgeon who will definitively fix what's going on.
